A method and a device for controlling an exhaust-gas aftertreatment system are known from German Patent 199 03 439, where a method and a device for controlling an exhaust-gas aftertreatment system are described, the exhaust-gas aftertreatment system including at least one catalytic converter, and a predefinable quantity of reducing agent being supplied to the exhaust-gas aftertreatment system as a function of the state of the internal combustion engine and/or the exhaust-gas aftertreatment system. The quantity of reducing agent supplied is determined on the basis of operating characteristics of the internal combustion engine, such as the rotational speed and the quantity of fuel injected as well as variables which characterize the state of the exhaust-gas aftertreatment system, e.g., the exhaust gas temperature upstream from, within, and/or downstream from the exhaust-gas aftertreatment system.
When using urea as the reducing agent, metering of the reducing agent is problematical. If too little reducing agent is supplied to the exhaust-gas aftertreatment system, the result is that nitrogen oxides are not adequately converted and then they enter the environment. If too much reducing agent is supplied, in particular in the case of metering a urea solution, unwanted substances, e.g., ammonia, will enter the environment. In order to prevent the emission of ammonia, the usual procedure tends to be to meter too little reducing agent. In other words, the quantity of reducing agent is predefined so that tolerances in the area of the internal combustion engine, the exhaust-gas aftertreatment system or the metering system for metering the reducing agent do not result in the presence of ammonia in the exhaust gas.